FIG. 1 shows a schematic depiction of a known mobile communications network 100. The network comprises a plurality of base stations 200 which define macrocells 210 within the network. Mobile terminals 400 within the macrocells 210 can communicate via the base stations 200. The base stations 200 will have backhaul connections (not shown) to allow for connections to the internet and other communications networks. The mobile communications network 100 further comprises one or more wireless access points 300 which define smaller cells 310 known variously as microcells, picocells and femtocells. The distinction between macrocells, microcells, and picocells is primarily one of scale, but femtocells typically have different backhaul arrangements to the other types.
Most base stations are installed, managed, and controlled by the network operator which also provides a dedicated transmission path back to the core network, controls the channels used by each base station to minimize interference between them, and maintains a “neighbor” list of other cells to which a handover is likely to be made. However, typical femtocells are installed, and powered, by an end user or business with less active remote management by the network operator, and are semi-autonomous, sensing from their immediate environment the best frequency and radio parameters to use. Backhaul connection is usually made through a public network, typically a DSL (digital subscriber loop) connection through the Internet.
For present purposes the backhaul arrangements are not significant, and the term “small cell” will be used to embrace any cell smaller than a macrocell.
It will be understood that in practice the edge of the area of coverage (“cell”) 210, 310 of each base station is not a clearly defined boundary, but signal quality and strength decline with distance from the base station, further attenuated by buildings, foliage etc. For most purposes, however, the cell can be considered as delineated by a boundary defined by a particular value for signal quality. Cells overlap at locations where signal quality from more than one base station exceeds that value.
The wireless access points 300 transmit their signals at lower power levels and thus the small cells 310 cover a significantly smaller geographical area than the macrocells 210. A small cell can be used to provide network coverage in an area which macrocells do not cover and for which it is not economic to use a macrocell to cover that area. Alternatively a small cell can be deployed within a macrocell to provide greater network capacity such that mobile terminals in range of such a small cell can communicate via the small cell, the macrocell or the small cell and the macrocell simultaneously. The wireless access points 300 will also have backhaul connections (not shown) to allow for connections to the internet and other communications networks. The base stations and wireless access points preferably transmit and receive signals conforming to the LTE (Long Term Evolution) standard but it will be understood that other data standards could be used.
The wireless access points used to provide the small cells may be located within areas in which significant numbers of users are expected, for example shopping centers, railway stations, etc. Alternatively, LTE functionality could be incorporated into devices such as the applicant company's “Home Hub” product, which incorporates the functions of xDSL modem, router and WiFi access point. In such a case, an LTE mobile terminal would be able to connect to the device using its wireless access point function and the data would be routed via the DSL link to a core network and then onwards to its destination. For such a wireless access point, the LTE will have a power output of approximately 27 dBm which is likely to mean that the small cell will have a usable range of approximately 30 m radius. As discussed above, such very small cells using DSL as a backhaul rather than a dedicated fixed network, are often referred to as “femtocells”. Unless the context requires otherwise, the term “small cell” in this specification embraces such femtocells.
LTE requires frequency synchronization among the access points, and the degree of accuracy depends upon the size of the cell—the effective area of coverage of the access point. Some modes of LTE operation also need timing phase synchronization, which means that all the access points need to be closely aligned in absolute time with respect to one another. FDD (Frequency Division Duplex) requires accurate frequency synchronization between small cells (between 100 and 250 parts per billion depending upon base-station class).
The use of TDD (Time Division Duplex) spectrum necessitates synchronization of base-stations in terms of timing phase because the transmit-receive switching of all the base-stations in the network needs to take place simultaneously in order to avoid intra-network interference.
When TDD is used in small cells that are deployed in houses, for example as part of a Home Hub femtocell, a major problem is that timing phase information cannot be passed down DSL links with sufficient accuracy. The cells need to be synchronization with one another to avoid problems. A known method of synchronization adjacent cells is for the cells to receive transmission from other cells and to co-ordinate their transmit-receive switching instants in time. However, such techniques are not effective when the first cell cannot receive signals from the second cell (and vice versa).
Such synchronization can be obtained, for example, by receiving a GPS (Global Positioning System) signal transmitted by a satellite, or by receiving a signal that is transmitted by a ground-based broadcasting station and that contains a reference time or frequency. However, when one or more base stations are located inside a building, receiving such synchronization signals is difficult, such that the synchronization is prevented or at least altered. Moreover, the base stations of a single-frequency network (SFN) are generally interconnected via a smaller-scale “backbone”, such as an ADSL (asymmetric digital subscriber line) connection, whose synchronization is not as precise as the synchronization required for SFN operation.
Failure to achieve synchronization can result in interference between the transmissions of the various base stations. LTE-TDD needs phase synchronization to within about 3 microseconds across any group of access points with overlapping coverage. Whether FDD or TDD, some advanced LTE features also need accurate phase synchronization, for example the LTE-A feature CoMP (Co-operative Multipoint) is a kind of network MIMO that needs +/−0.5 microsecond timing phase accuracy across the group.
Synchronization is also required to ensure a successful handover of a mobile terminal from the coverage of one base station to that of another.
One method of achieving time phase synchronization would be to equip every cell with a very accurate clock, but this would be prohibitively expensive. Another is to monitor other networks, such as GPS or some other broadcast clock signals, but this can be unreliable when inside buildings, where line-of-sight to the transmitter of the broadcast signal may not be available, as a path length extended by as little as 150 meters by multipath effects (such as reflection of neighboring buildings) would delay the time signal by more than the required +/−0.5 microsecond timing phase accuracy.
A further option is to provide both frequency and time phase information over the backhaul network, and there are solutions for this, such as NTP, RFC1588v2 or NTR, usually used along with SynchE. Whereas frequency synchronization can be cheaply provided using NTP, and this is standard practice for 3G small cells, it is very much more costly to provide time phase information over the backhaul if there is a segment of the path that uses xDSL technology. This presents a significant difficulty for femtocells within houses, since xDSL is used in the majority of broadband installations.
One known approach to this problem of timing phase synchronization, is to arrange for small cells to monitor each other off-air by means of the receivers fitted to them. They can demodulate one another's transmissions and run an algorithm within themselves, to align the time phase with each other. However, it is common for small cells to not be located within range of each other's transmissions. In the arrangement shown in FIG. 1 this can be resolved by using mobile units 400 located in an area 500 within range of two or more base stations 200, 210, 300 to relay the synchronization data. However, this is not always possible.
FIG. 2 shows a schematic depiction of such a situation, showing part of a mobile communications network with a first wireless access point 300a and second wireless access point 300b. The first wireless access point 300a defines a first small cell which has an associated first coverage area 310a. Similarly, the second wireless access point 300b defines a second small cell which has an associated second coverage area 310b. It can be seen from FIG. 2 that the first wireless access point 300a is outside the second coverage area 310b and that the second wireless access point 300b is outside the first coverage area 310a. Moreover, unlike in FIG. 1, there is no overlap area 500 which is part of both the first coverage area 310a and the second coverage area 310b. 
Referring to FIG. 1, assume that a first wireless access point 200 is already synchronized, a mobile terminal 400 within its range of coverage 210 will become synchronized with the first wireless access point 200. If the mobile terminal 400 is also within range of a second wireless access point 300 which cannot receive synchronization data from the network or another access point, the second wireless access point 300 can detect the transmissions from mobile terminal 400 and thus receive the necessary synchronization data from the mobile terminal. Thus, it can be seen that the mobile terminal 400 acts as a bridge, with the synchronization data being transmitted from the first wireless access point 200 to the second access point 300, via the mobile terminal 400. French Patent specification FR2972322 describes a system operating in this way.
However, a, common problem with indoor small cells is that there is little or no overlap between them, or between a small cell and the macrocell coverage available outside the building. Small cell coverage is usually optimized to cover the individual rooms of a building and any coverage of the threshold or entrance hallway, or the area immediately outside the entrance, is likely to be incidental and fortuitous. Even where the threshold of the building does represent an overlap between small cell and macrocell coverage, mobile units are only within the overlap zone very briefly, for example when entering or leaving a building.
This is, of course, precisely the time that synchronization is required to be already in operation, as the mobile unit hands over between the outdoor macrocell and the indoor small cell.
The reference discussed above attempts to solve this by weighting times taken by base stations from mobile units according to a number of measures relevant to the reliability of the time value received from the mobile unit. However, as base stations time bases may drift relative to each other, the system is not stable and requires constant updating.